Effects of hydroxyapatite reinforcement on the architecture and mechanical properties of freeze-dried collagen scaffolds.

Freeze-dried collagen scaffolds reinforced with hydroxyapatite (HA) are of clinical interest for synthetic bone graft substitutes and tissue engineering scaffolds, but a systematic evaluation of the effects of the HA reinforcement weight fraction and morphology on the mechanical properties is lacking. Therefore, freeze-dried collagen scaffolds were reinforced with either HA whiskers or an equiaxed HA powder at 1:1, 1:2, or 1:4 collagen:HA by weight (which corresponded to approximately 28, 44, and 61 vol% HA, respectively) to investigate the effects of the HA reinforcement weight fraction and morphology on the architecture and compressive mechanical properties. All scaffolds exhibited a highly elongated linear pore structure containing 90%-96% porosity, which decreased with increased HA content, and a pore width of ∼50 μm. HA reinforcement resulted in up to a ten-fold increase in compressive modulus at high reinforcement levels (∼200 kPa at 1:4 collagen:HA by weight) compared to scaffolds with no reinforcement or low reinforcement levels (∼20 kPa at 1:1 collagen:HA by weight). This effect could not be explained by the concomitant decrease in the scaffold porosity (from 95% to 90%) with HA reinforcement, which could only account for up to a two-fold increase in compressive modulus. At moderate reinforcement levels (1:2 collagen:HA by weight), HA whisker reinforced scaffolds exhibited a nearly four-fold greater modulus compared to the equiaxed HA powder, while there were no differences with the HA reinforcement morphology at high and low reinforcement levels. Therefore, the elongated morphology of HA whiskers enabled a reinforcing effect at a lower level of reinforcement compared to a conventional, equiaxed HA powder.